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Relative formula mass and conservation of mass
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Chemists weigh things to understand reactions. Adding up atomic masses tells you how heavy a compound is.
Relative formula mass (M_r)
Add up the relative atomic masses (A_r) of every atom in the formula.
H2O: (2 × 1) + 16 = 18
CO2: 12 + (2 × 16) = 44
H2O: (2 × 1) + 16 = 18
CO2: 12 + (2 × 16) = 44
Conservation of mass
Atoms are not made or destroyed in a reaction, so the total mass of reactants = total mass of products.
If the mass seems to change, a gas has usually escaped or been taken in from the air.
If the mass seems to change, a gas has usually escaped or been taken in from the air.
Percentage by mass
% of an element = (A_r × number of atoms) ÷ M_r × 100
% of C in CO2 = 12 ÷ 44 × 100 = 27.3%
% of C in CO2 = 12 ÷ 44 × 100 = 27.3%
10.0 g of calcium carbonate is heated. It forms 5.6 g of calcium oxide and carbon dioxide gas. What mass of carbon dioxide forms?
- Mass of reactants = mass of products.
- 10.0 = 5.6 + mass of carbon dioxide
- mass of carbon dioxide = 10.0 − 5.6 = 4.4 g
Answer: 4.4 g
M_r is the sum of the A_r values in the formula. Mass is conserved: reactants and products have the same total mass. Gas escaping makes mass seem to drop.
The interactive lesson includes the diagrams for this topic.
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